The disclosure herein relates generally to testing semiconductor devices and more particularly to a method and apparatus for testing and measuring the current drawn by Integrated Circuits (IC) after initialization.
A single point current measurement analysis is typically performed to test an IC for defects according to the known method called the I.sub.DD test, which is well known in the art. The known method involves initializing the device under test (DUT) and then allowing the current to reach a stable value. When an "ideal" device reaches a stable value, the logic circuits, which are contained in the ideal device, draw negligible or no current. Once the DUT has reached a stable value, a measurement is taken to determine how much current, if any, the logic circuits are drawing. This measurement is compared to standard values to determine if the DUT is defective.
Before taking a measurement the DUT must be initialized. The initialization process involves powering up the DUT and inputting a series of different stimulus vectors to the inputs of the DUT. The vectors, which are binary strings, are used to create different logical states in the DUT.
After the last initialization vector is supplied, the DUT is given a predetermined period of time to reach a "static" state; this time is typically 1 msec. After the predetermined period of time has elapsed the DUT will be at a stable value. Once the DUT reaches the stable value, a single measurement, called the stable value current (I.sub.DDQ), is taken using a Precision Measurement Unit (PMU). The PMU measures the current being drawn by the logic circuits. Only one measurement is taken once the predetermined period of time has lapsed. This measurement is then compared to a standard value designated as the "good/bad limit" for that particular device. Based on the result of this comparison the DUT will either be good or defective.
When using the known method, it is possible that a defective DUT will not be detected because during the settling period the PMU does not take any measurements. The known method of testing ignores the behavior of the DUT during the settling period. The DUT is given a predetermined period of time to settle and reach a stable value. Once the predetermined period of time has elapsed and the DUT has reached a stable value, the PMU will measure the current drawn by the DUT. According to the known method of testing, no measurements occur during the settling period. The PMU takes the measurement at the stable value and this result is compared to a standard value. Thus, the measurement is only an indication of the DUTs behavior at one point in time; it does not indicate how the DUT behaved during the settling period. Given sufficient time, it is possible for a defective DUT to settle to the same level as a good device. Consequently, a measurement taken after the DUT reaches a stable value is not the best indicator of the DUT's quality.
If only a single measurement is used to determine the quality of a DUT, a defective DUT may not be detected because the known method of testing ignores the DUT's behavior during the settling period. Additionally, this wastes tester time because defective devices are not detected.
Therefore, what is needed is a method and apparatus that measures the response of the DUT over a period of time, including the settling period, to create a current waveform of the DUT's performance.